Studies of porphyric cattle are continuing. These involve development and use of improved methods for assay of porphyrins (including isomers) and of enzymes involved in porphyrin biosynthesis. Repeated analyses of blood CO and fecal urobilinogen will be made in these photosensitive animals to evaluate factors such as light exposure which influence red cell destruction. The possible therapeutic value of compounds which inhibit porphyrin biosynthesis will be investigated. Attempts will be made to prepare a current national census of cattle known to have (or to be heterozygous for) porphyria.